1. Field of the Invention
The present invention relates to a polyester multilayer sheet and a process for preparing the same suitable for a window glass substitute in buildings, transparent clothing material for agriculture and several other uses based on its excellent impact strength, weatherability, and transparency.
More particularly, the present invention relates to a polyester transparent multilayer sheet, having excellent impact strength and weatherability characterized being coextruded in turn with more than 3 layer of sheet using more than 2 kinds of copolyester resin which are composed of 5-99 mol % of ethyleneterephthalate and 1-95 mol % of ethylenenaphthalate or ethyleneisophthalate and have at least 0.005 dl/g of inherent viscosity difference between adjacent layers, and the higher inherent viscosity of resin is arrayed at the outer layer.
Additionally the present invention relates to a process for preparing polyester multilayer sheet having an outer layer and inner layers having excellent impact strength and weatherability, characterized by coextruding more than 2 kinds of copolyester resin, which are all composed of 5-99 mol % of ethyleneterephthalate and 1-95 mol % of ethylenenaphthalate or ethyleneisophthalate and have at least 0.005 dl/g of inherent viscosity difference between adjacent layers on more than 3 layers of multilayer sheet which have been arrayed in turn by high inherent viscosity resin and low inherent viscosity resin, wherein said outer layer of the multilayer sheet has higher inherent viscosity than the adjacent inner layer.
2. Description of Prior Art
Efforts at alternating constructional glass with transparent plastic have been unceasingly made since transparent resins such as Polystyrene, Acryl and Polycarbonate were invented. It had been anticipated that several plastic materials could replace glass in various applications such as lighting window based on their merits of lightness in weight and impact-resistance.
Such plastic material, however, is not practical as a substitute for glass due to its low weatherability. In other words, glass made of inorganic material has almost perfect weatherability, but plastic material made of organic resins are vulnerable to UV rays, has too low weatherability to secure a position as a general glass substitute entirely.
Though Acryl resin and Polycarbonate are currently routinely used instead of glass in certain applications, Acryl is weak in its impact strength, and Polycarbonate is vulnerable in its weatherablilty. Therefore, these two material can not satisfy the two most important properties of a glass substitute at the same time.
In the mean time, polyester resins represented by polyethyleneterephthalate or polyethylenenaphthalate, have lots of merits required of transparent sheet for glass substitute in that:
they have excellent mechanical properties, transparency, and durability; PA1 they are able to form thicker than 3 mm transparent sheet, required for a glass substitute in soundproofing walls because the polyester resin can be properly delayed in its crystallization by copolymerization or blend; PA1 they are 20-25 times the impact strength of Acryl, and 2-3 times the weatherability of Polycarbonate; and PA1 their prices are similar or cheaper than Acryl. PA1 On the whole repeating unit of copolymerized polyester resin, pour 5-99 mol % of dimethylterephthalate (more preferably 85-99mol %), 1-95 mol % of dimethyl-2,6-naphthalene dicarboxylate, dimethylisophthalate or isophthal acid (more preferably 1-15 mol %), and 150-250 mol % of ethylene glycol into a reactor; PA1 Obtain some low molcular weight copolymer by a transesterification reaction under certain catalyzers such as Zinc, Manganese, Cobalt, Magnesium, or Calcium; and PA1 By depressurizing below 1 Torr, perform a polymerization reaction at 270-300.degree. C. by applying a polymerization catalyst such as Antimony, and Germanium.
Actually, some polyester resin products which are suitable for preparing various kinds of transparent sheets have been invented by researchers in several countries and are on the market. These are known as amorphous polyester.
This amorphous polyester, however, is weakened by UV rays if used outdoors for a long time due to being relatively more vulnerable to weatherability than Acryl. This phenomenon is because the polyethyleneterephthalate resin allows transention of UV rays over 320 nm.
In practice, UV stabilizer can be added in order to cover this vulnerability, however, their value as glass substitutes which requires transparency with no color, is impaired due to deteriorated color.
For these reasons, and on behalf of developing certain polyester resins having excellent weatherability and transparency, it is essential to produce certain polyester resins fundamentally able to protect against UV rays fundamentally.
On the other hand, AMOCO Technical Magazine (FA13a, 1994) suggests a method of preparing mono-layer transparent sheets comprising 2-3 times the weatherability of polyethyleneterephthalate by applying a copolymer of polyethyleneterephthalate and polyethylenenaphthalate as raw material in order to the UV protecting property of polyethylenenaphthalate. This copolyester resin could be applied to a heat-resistance container, gas-proof container, refillable/returnable container, medicine container and UV protecting container based on its excellent heat-resistance, protecting property of oxygen, carbon dioxide, vapor and UV, chemical-resistance and hydrolysis-resistance.
Since the price of polyethylenenaphthalate itself is fairly expensive, economic efficiency, despite of the improved UV protecting property, shall be deteriorated if polyethylenenaphthalate is used too much. Meanwhile, if polyethylenenaphthalate is used too little, the resin looses its UV protecting property and is weakened by UV light.
Accordingly, the purpose of present invention is to provide polyester multilayer sheet suitable as a window glass substitute in buildings or houses based on its excellent impact strength, weatherability, and transparency with cheap production cost.